Congestive heart failure is a chronic, degenerative condition that impairs the heart's ability to pump blood at normal filling pressures to adequately meet the energy requirements of the body. It is estimated that 4.9 million Americans suffer from various degrees of congestive heart failure (CHF), with about 400,000 new cases identified each year. Heart failure is the most common diagnosis in hospital patients over the age of 65, and it carries a mortality rate higher than that for malignant tumors. One in five CHF patients dies within one year of diagnosis and only 15% survive more than 10 years.
There is no cure for CHF short of a heart transplant. However, advances in pharmacology have provided improved treatment programs. Multidrug treatment regimens that include diuretics, vasodilators and inotropic agents such as angiotensin-converting enzyme (ACE) inhibitors, can slow the progression of CHF and reduce the number of acute episodes. However, treatment remains directed at symptoms and is most effective in the early stages of CHF.
In later stages of the disease, mechanical devices can play an important role. Specialty left ventricular pacemakers can improve the heart's function as a pump, while cardiac assist devices may be used to help support the failing heart. These devices primarily address the needs of approximately 25% of CHF patients.
The increase in CHF has inspired researchers to look for new ways to treat this disease, leading to the development of drugs, surgical procedures and mechanical pumps to assist heart function, which either prevent or at least delay progression of the condition. One such approach involves implantation of an artificial heart, comprising, a diaphragm pump, an example of which is described in U.S. Pat. No. 4,468,177. The pump disclosed in this patent comprises two chambers which are driven in a “push-pull” manner so that the volumes of each of the two chambers are alternatively enlarged and reduced and, further, both chambers are in the path of fluid through the pump. However the use of this, or a similar, pump arrangement for total heart replacement or heart assistance would require an extremely reliable, durable and portable power supply. Although mechanical pumps have been constructed and tested, they have not been found to be adequate for long-term treatment of heart failure.
Chronic heart failure is characterized by changes in the biochemical composition of the ventricular wall, the orientation of cardiac muscle fibers, and the geometry of the ventricular chamber. Remodeling is the term commonly used to describe these facets of cardiac adaptation in disease. While some aspects of the remodeling process may be beneficial to overall heart function to this disease, other aspects are likely to be maladaptive over extended periods. One notable maladaptive aspect of remodeling is left ventricular dilatation, which is characterized not only by an increase in ventricular end diastolic volume but also by shifts of the end diastolic pressure volume relation toward larger volumes. Ventricular dilation increases wall stress and imparts mechanical disadvantage to the myofibrils; therefore, it is a critical event in the disease process.
A decrease in the internal volume of the damaged left ventricle will increase its efficiency because the amount of oxygen consumed by the myocardial muscle as it pumps is related to the wall tension developed during ventricular contraction. The wall tension, in turn, is proportional to the fourth power of the diameter of the ventricular cavity. Therefore, at a given smaller diameter, less work will be used by the muscle to pump a given volume of blood against a given pressure.
Cardiac output has been improved surgically by reducing the volume of a diseased left ventricle. Removal of the non-functioning aneurysmatic segment of the heart muscle has improved the hemodynamic situation by changing the geometry of the left ventricle, leading to enhanced cardiac output. However, this procedure, generally referred to as the Batista technique, is associated with considerable operative risk.
An improvement upon this operation is disclosed in U.S. Pat. No. 5,738,626. In this modified surgical procedure, which includes elements of the other approaches as well, the dilated left ventricle is first reduced in size via myocardial resection and then the heart is supported and assisted by the attachment of a cardiomyoplasty muscle wrap. Cardiomyoplasty is a term used to describe a procedure in which the sheet-like latissimus dorsi muscle is mobilized (while maintaining its vascular supply) and inserted into the mediastinum through a lateral thoracotomy. After pericardiectomy, the muscle is wrapped around both the left and right ventricles. Several weeks are provided for surgical recovery and electrical muscle conditioning, and thereafter the muscle is paced by burst stimulation synchronized to every other cardiac systole. Chronic repetitive stimulation induces biochemical and physiological transformations in the muscle, altering its characteristics toward those of cardiac muscle. These changes include fatigue resistance, prolonged contraction duration, diminution in size, and reduced maximal power.
The principal mechanism by which cardiomyoplasty has been assumed to assist the failing heart is augmentation of systolic ejection by active squeezing of the ventricles. While human studies have reported improvement in the clinical symptoms of patients undergoing this therapy, evidence for active systolic assist has been inconsistent. The results have led some to speculate that more passive external constraining effects of the muscle wrap maybe a source of benefit.
Surgical modes of intervention for the treatment of heart failure include: stiffening zones of acute infarction, for example by directly injecting glutaraldehyde into the affected tissue; wrapping a skeletal muscle (stimulated electrically) around the heart to augment ventricular contractility; and applying an epicardial marlex mesh to support the weakened and distended left ventricle.
A disadvantage common to cardiomyoplasty procedures, which is a consequence of the use of skeletal muscle as the ventricular wrap, is that the tissue wrap fatigues upon repeated stimulation at normal heart rates. Therefore, a period of time exceeding several weeks is required after this operation to condition the skeletal muscle, transforming it into a different tissue, rich in mitochondria and adapted to withstand the repeated stimulation with much less fatigue. During this period a ventricular assist pump may be implanted within a hole cut in the ventricle. Eventually, this assist pump will be removed.
Today, ventricular assist devices (VAD) designed to support a failing heart, represents the most important device technology for treating CHF. These devices are manufactured by Thermocardiosystems (TCS: Woburn, USA), Thoratec (Berkeley, USA), Abiomed (Danvers, USA), and Novacor (Santa Ana, USA).
Some heart remodeling techniques are also under development. Cardio Technologies (Pine Brook, N.J.) is developing a cardiosupport system, that acts via direct mechanical ventricular activation which mimics open chest resuscitation. The device squeezes the heart by applying pressure to a cup-like component placed around the heart.
Acorn Cardiovascular (New Brighton, Minn., USA) envisions a completely new technology for the treatment of CHF. The company is developing a passive device made of biocompatible materials that is placed around the heart via minimally invasive surgical techniques. The device is able to reduce the size, preventing the heart from enlarging further. Unlike surgical reduction techniques (Batista techniques), the product does not require the removal of any heart muscle.
Additional developments are illustrated in the following patents.
U.S. Pat. No. 5,749,855 issued to Reitan et al. describes implantable catheter pump including a drive cable, with one end of the drive cable being connectable to a drive source, a collapsible drive propeller being adjustable between a closed configuration in which the collapsible drive propeller is expanded so as to be operative as an impeller, and a sleeve extending between one side of the collapsible drive propeller and the other side of the collapsible drive propeller.
U.S. Pat. No. 5,908,378 issued to Kovacs et al. describes a cardiac assist device comprising of an outer shell and a diaphragm, formed of polyurethane copolymer.
U.S. Pat. No. 5,824,071 describes an apparatus for treatment of ischemic heart disease by providing transvenous myocardial perfusion.
U.S. Pat. No. 5,798,102 describes a method of treating cardiomyoplasty with a composition comprising beta-amyloid, streptolysin O and growth hormone.
U.S. Pat. No. 5,702,343 describes a cardiac reinforcement device and method of treatment of cardiomyopathy. This device provides for reinforcement of the walls of the heart by constraining cardiac expansion, beyond a predetermined limit, during diastolic expansion of the heart. This device is applied to the external wall of the heart and surrounds the complete cardiac wall.
U.S. Pat. No. 5,738,626 discloses a cardiomyoplasty procedure comprising excision of the tissue of the myocardium and replacing it with a muscle wrap. This muscle is conditioned with a support device.
U.S. Pat. No. 5,848,962 issued to Feindt et al. describes a half shell which is placed against the ventricle and an external filling unit which compresses the shell synchronously with cardiac activity, enhancing the ejection fraction of the ventricle.
U.S. Pat. No. 5,800,528 describes a passive girdle which is wrapped around a heart muscle which constrains the dilatation during diastole. The girdle is formed of a material and structure that does not expand away from the heart but may, over an extended period of time be decreased in size as dilatation decreases.
U.S. Pat. No. 5,282,849 issued to Kolff et al. describes a ventricle assist device with volume displacement chamber.
U.S. Pat. No. 4,902,291 issued to Kolff et al. describes a collapsible artificial ventricle and pumping shell.
Nevertheless, none of the above approaches is wholly satisfactory for the treatment of congestive heart failure. Therefore, there is a need for additional or alternative methods for treatment heart pump failure.